1. Field of the Invention
The present invention relates to control units and, more particularly, to switching potentiometer current control units.
2. Technical Considerations and Prior Art
Current control units, for example, light dimming units for controlling the brightness or intensity of a lamp load connected in series with the unit across a supply line, commonly include switching potentiometers which perform two functions. The switching portion of the switching potentiometer connects the dimmer unit to the supply of line voltage while the potentiometer portion of the switching potentiometer provides control of the current to the lamp load, for example, through an a.c. phase control circuit.
There are basically two types of switching potentiometers commercially available. These may be classified as the rotary type and the push-button type. Push-button type switching potentiometers may employ either push-pull or push-push (reciprocating) switches and account for the majority of sales of dimmer units since they permit the unit to be switched on by pushing (push-pull) or pushing and releasing (push-push) a resistance control knob and rotating the control knob until the desired level of brightness is obtained. Once a desired level of brightness has been selected the unit may be turned off simply by actuating the control knob by pulling it outwardly (push-pull) or by pushing and releasing it (push-push). Thereafter the desired brightness setting can be provided without any need for re-adjustment.
The principal drawback of push-button type dimmer units is that with minimum resistance settings, i.e., relatively high brightness settings, arcing takes place at the switch contacts when the switch is operated. This, of course, reduces the life of the contacts and causes current surges through the load which may also reduce the effective life of the load.
Rotary type switching potentiometers, on the other hand, are typically, though not always, at maximum resistance when the load is switched. They are typically operated by rotating the control knob to switch power to the device and then continuing to rotate the control knob until the desired brightness setting is reached. When it is desired to turn the unit off the control knob is rotated in the opposite direction to reduce the brightness of the load to a minimum before switching takes place. While the life of the contacts and the life of the load are thus extended it is necessary to reset the desired level of brightness each time the unit is turned on.
Rotary type switching potentiometers of the type set at minimum resistance rather than maximum resistance after switching possess the further disadvantage that arcing takes place whenever the unit is switched on since the load is thus provided with maximum current and thus a maximum brightness which may be dimmed by a further rotation of the control knob.
It would therefore be desirable and advantageous to devise a switching potentiometer control unit for light dimming and similar applications which avoids the disadvantages of both conventional push-button and rotary type switching potentiometers. Thus, it would be desirable to devise a switching potentiometer which could both be set to a desired brightness level for controlling the lamp load, that could be switched on and off without changing the brightness level such that a desired level can be maintained between operations of the switching portion of the switching potentiometer and which could assure that minimum arcing takes place at the switch contacts when the unit is turned on or off.